The use of 13C-labeled tracers in nutritional status assessment has recently received increased attention. Breath tests usung low-cost 13C-labeled tracers are emerging, along with breath tests in general in metabolic monitoring, as useful clinical tools in the study of nutritional disorders inn a manner compatible with an out-patient setting. In such tests, the 13CO2/12CO2 is measured in breath following 13C-labeled tracer ingestion: the rate and amount of 13CO2 release can be used as a quantitive measure to discriminate between normals and patients. A limiting factor to the routine use of such patient-easy tests is the absence of a convenient, low-cost analytical tool for measuring 13CO2/12CO2 in breath. The proposed Phase I program will be directed toward providing the data required to initially validate the use of a new IR-based instrument in direct measurement of 13CO2/12CO2 in breath. This new ratio infrared spectrometer (RIS) uses a highly reflective light pipe as a low volume (5cc) sample cell. The IR-source is directed at a slight angle to the axis of this light pipe; the light pipe thus also serves as a long-pass White cell without the bulk and cost normally asssociated with such a cell. A beam splitter and three pyroelectric detectors are used in the RIS to give simultaneous measurement at 13CO2, 12CO2 and reference wavelenghts. Initial work done by TRACER with the RIS has shown its repeatability in sample measurement to be 1.3 APE for 13C over the CO2 range of 2-6 vol%. In Phase I, the RIS will be modified to permit use of a low volume (25 cc) gas syringe as a sampling device. Using this modified device, correction algorithms will be developed to account for the observed non-linearity in CO2 response; these algorithms will be verified using reference samples analyzed by a ratio mass spectrometer (RMS). Initial validation of the RIS as the instrument of choice in breath tests for screening in nutritional status assessment will be undertaken using breath samples from normals and patients. Successful completion of Phase I will lead to a Phase II program in which in-field RIS validation is carried out in collaboration with clinicians presently evaluating breath tests in nutritional status assessment.